1. Technical Field
The present invention relates to the production of a customized progressive power ophthalmic lens, in other words, a progressive power spectacle lens which is intended for an identified wearer.
2. Description of Related Art
Progressive power lenses have been used for many years to correct an ametropy of a wearer in a manner that is suited both to far vision and near vision. For this, the lens has optical power values that are variable along a meridian line, between a reference direction for far vision and a reference direction for near vision. The optical power values for these two reference directions are determined from a prescription which is prepared for the wearer. Usually, the prescription indicates an optical power value for far vision and an addition value. The optical power value of the lens that is appropriate to the wearer to correct his sight in near vision conditions is equal to the sum of the optical power value which is prescribed for far vision and the prescribed addition value. The lens which is supplied to the wearer is produced in such a way as to produce substantially the optical power value which is thus calculated for near vision and the optical power value which is prescribed for far vision, respectively for the two reference directions for near vision and for far vision.
It is known that a progressive power lens exhibits, in a manner which is inherent in its principle, an unintentional astigmatism which may be distributed in lateral regions of the lens, so as to interfere with the vision of the wearer as little as possible. This distribution of the unintentional astigmatism can be performed by favoring a wide channel without astigmatism, between the reference directions for the far and near visions. However, the unintentional astigmatism is then greater towards the lateral edges of the lens. Alternatively, a channel without astigmatism which is narrow makes it possible to reduce the unintentional astigmatism values in the lateral regions of the lens.
It was then proposed to adapt the distribution of the unintentional astigmatism according to the propensity of the wearer to prefer to turn the head or the eyes horizontally when looking successively in two different directions situated at one and the same height. Such an adaptation of the design of the progressive power lens makes it possible to reduce the nuisance to the wearer caused by the unintentional astigmatism. It therefore involved a customization of the progressive power lens according to the wearer, over and above matching the lens to the optical prescription prepared for the wearer.
It is also known to adapt a progressive power lens to suit the vertical movements of the head of the wearer. This is a further customization of the progressive power lens, which is different from the preceding one based the horizontal head movements. The aim of such a customization, which takes account of the changes of inclination of the head in a vertical plane, is to adjust the variation of the optical power of the progressive power lens along the meridian line. In this way, the optical power is adapted to the distance of an object being looked at regardless of the angular height of the direction in which the object is located in front of the wearer. To this end, the document EP 1 591 064 for example proposes a device which makes it possible to determine the variations of inclination of the head of the wearer when looking alternately in the reference direction for far vision and that for near vision.
However, the methods for customizing a progressive power lens proposed before the present invention, to take account of the movements of the head of the wearer in a vertical plane, are limited to near vision conditions. Intermediate vision conditions between the near and far visions are sometimes taken into account. In the context of the present description, near vision conditions correspond to an object being looked at which is situated less than 40 cm (centimeters) from the eyes of the wearer, and far vision conditions correspond to an observation distance which is greater than 120 cm. The customization of the progressive power lens which is done according to some of these prior methods consists in adjusting the height of the reference point for near vision inside the lens. In this way, a possible time for the wearer to become accustomed to the progressive power lens can be reduced.
Furthermore, in these prior customization methods, the wearer is always static in the observation conditions which are reproduced to determine his propensity to favor moving the head or the eyes. In other words, these prior methods for customizing a progressive power lens do not take any account of the movement of the wearer relative to the object being looked at.
Now, it has become apparent that a wearer of progressive power lenses customized for near vision can still be hampered in circumstances which correspond to far vision. The variable visual correction which is obtained by the progressive power lens may in fact be unsuited to the vertical head movements, when the wearer is looking at objects which are distant in the reference direction for far vision and then in variable directions below this reference direction. The result may therefore be a nuisance to the wearer during a period of becoming accustomed to a new progressive power lens.
Furthermore, it has been found that the vertical head movements which are performed by the wearer to look at distant objects, are different when the wearer is immobile or when he is moving. This difference is due to a spontaneous anticipation of the wearer relative to his actual movement. Such behavior occurs, for example, when the wearer is walking in the street and alternately looking at the ground in front of him and at a distant landscape.